1. Field of the Art
The present invention relates to a process of manufacturing a structural body for an automotive vehicle. More particularly, the invention is concerned with the process of manufacturing the structural body having a high degree of rigidity, without suffering from deformation or distortion of the body.
2. Discussion of the Related Art
In general, a hollow or tubular member used as a front pillar or a center pillar of an automotive vehicle has an enclosed inner space. The wind may pass through the inner space, thereby causing wind noise. Further, the vibration noise of the engine may be transmitted from the engine room to the interior of the vehicle through the inner space of the hollow member. Thus, the inner space in the hollow member undesirably leads to generation of noise in the interior of the vehicle. Conventionally, to deal with the above problem, the inner space in the hollow member is filled with a foamed body for preventing the wind from passing through the inner space and inhibiting the transmission of the vibration noise to the interior of the vehicle, so as to improve the sound insulation characteristics of the vehicle.
In recent years, the hollow member is filled with the foamed body in an attempt to increase the rigidity of the structural body of the vehicle such as pillars, and to improve the amount of absorption of energy generated upon collision of the vehicle, while at the same time meeting other demands, such as reduction of the weight of the vehicle and cost of its manufacture, as well as reduction of the noise conventionally generated in the interior of the vehicle. In view of the above, the hollow member is usually filled with a highly rigid or high-rigid foamed body which is obtained by foaming a hard foamable material. In the specification, the hard foamable material means a foamable material to form a highly rigid foam.
There are various known methods for filling the inner space in the pillar member with the foamed body. For example, the inner space is filled with a foamed body by inserting the foamed body having a configuration corresponding to that of the inner space, through an opening formed through the hollow member. Alternatively, a foamable material which is capable of foaming and expanding upon heating is disposed in the inner space as a spot sealing member, and is foamed by heat generated in the process of manufacture of the pillar member. For instance, the foamable material is foamed to fill the inner space, in a furnace in which the pillar member is baked for electro-deposition coating. When the inner space is filled with the foamed body according to the former method as described above, the foamed body having the same configuration as the inner space is inserted into the inner space through the opening of the pillar member. In this case, it is difficult to completely and uniformly fill the entire space in the pillar member with the foamed body, without an air gap adjacent to the inner surfaces of the pillar member. Moreover, since the opening through which the foamed body is inserted is formed by punching out a portion of a steel plate which constitutes a part of the pillar member, the worker may have his hand injured by contact with the edge portion of the opening formed by punching, upon inserting the foamed body through the opening.
When the inner space is filled with the foamed body obtained by foaming a foamable material, in particular, a hard foamable material, according to the latter method as described above, an excessively high degree of foaming pressure is likely to act on the steel plates which form the hollow member in the process steps of foaming and curing of the foamable material, whereby the steel plates undesirably tend to be deformed. More specifically described by reference to FIGS. 1(a)-1(c), when the hollow member is filled with the hard foamable material, a hard foamable material 4 having a rectangular cross sectional shape is initially disposed in the inner space of the hollow member in the form of a pillar 2, such that the foamable material 4 is fixed to the pillar 2, as shown in FIG. 1(a). Then, the hard foamable material 4 is heated for foaming, whereby the hard foamable material 4 expands in the inner space of the pillar 2 so as to completely fill the entire inner space, as shown in FIG. 1(b). If the foaming of the hard foamable material 4 proceeds, the hard foamable material 4 continues to expand in the inner space of the pillar 2. It is noted that the hard foamable material 4 has a high degree of expansion or foaming ratio, so as to expand to such an extent that the foamable material 4 completely fills the inner space of the pillar 2 when it is foamed. Accordingly, if the foaming process of the foamable material 4 continues even after the inner space of the pillar 2 has been filled with the expanded foamable material 4, the foamable material 4 is prevented from further expanding within the inner space of the pillar 2. In this case, the foaming pressure of the foamable material 4, i.e., an expansion force of the foamable material 4, acts on the inner surfaces of the pillar 2 to an excessive extent in the directions indicated by arrows in FIG. 1(b). Thus, the steel plates which form the pillar 2 may be undesirably deformed, resulting in deformation of a structural body 8 as shown in FIG. 1(c) which is filled with a highly rigid foamed body 6 obtained by foaming and curing of the hard foamable material 4.
Since the conventional foamable material generally takes the form of a sheet-like member having a relatively large thickness as shown in FIG. 1(a), this foamable material cannot be heated uniformly, namely, the heating temperature may considerably differ at its outer surfaces and at its inner or center portions. Thus, the conventional foamable material is less likely to be foamed with high stability. That is, the hollow member cannot be sufficiently filled with the highly rigid foamed body. Namely, some local portions of the foamable material may not be fully foamed due to insufficient heating. If the expansion ratio of the foamable material is raised in an attempt to eliminate the above drawback, the inner space of the hollow member can be completely filled with the highly rigid foamed body. On the other hand, however, the foaming pressure undesirably tends to be excessive, whereby the structural body 8 obtained is likely to be deformed as described above.